The present invention relates to micro-electro-mechanical devices (xe2x80x9cMEMs devicesxe2x80x9d) and, in particular, to improved MEMs devices held together by resilient packaging. The inventive MEMs devices are particularly useful as movable mirror devices for beam steering in optical communication systems.
MEMs devices are of considerable importance in optical communication systems. In one important application, a MEMs device provides a two-dimensional array of movable components, such as mirrors, sensors, or even mechanical paddles. Each movable component in the array can be electrically moved.
A typical MEMs mirror device comprises an array of metal-coated silicon mirrors, each mirror movably coupled to a surrounding silicon frame via a gimbal. Two torsional members on opposite sides of the mirror connect the mirror to the gimbal and define the mirror""s axis of rotation. The gimbal, in turn, is coupled to the surrounding silicon frame via two torsional members defining a second axis of rotation orthogonal to that of the mirror. A light beam can therefore be steered in any direction.
Electrodes are disposed in a cavity underlying the mirror and the gimbal. Voltages applied between the mirror and an underlying electrode and between the gimbal and an electrode control the orientation of the mirror. Alternatively, in slightly modified arrangements, an electrical signal can control the position of the mirror magnetically or piezoelectrically.
FIGS. 1(a) and 1(b) illustrate conventional optical MEMs devices and their application. FIG. 1(a) shows a typical prior art optical MEMs mirror structure. The device comprises a mirror 1 coupled to a gimbal 2 on a polysilicon frame 3. The components are fabricated on a substrate 4 by micromachining processes such as multilayer deposition and selective etching. After etching, mirror assembly (1,2,3) is raised above the substrate 4 by upward bending lift arms 5 during a release process. The mirror 1 in this example is double-gimbal cantilevered and attached onto the frame structure 3 by springs 6. The mirror 1 can be tilted to any desired orientation for optical signal routing via electrostatic or other actuation with electrical voltage or current supplied as to electrodes 7 from outside. The light-reflecting surface of mirror 1 comprises a metal-coated polysilicon membrane, which is typically of circular shape. The metal layers 8 are deposited by known thin film deposition methods such as evaporation, sputtering, electrochemical deposition, or chemical vapor deposition.
FIG. 1(b) schematically illustrates an important application of optical MEMs mirrors as controllable mirror arrays for optical signal routing. The cross connect system shown here comprises optical input fibers 10, optical output fibers 11 and an array of MEMs mirrors 1 on a substrate 4. The optical signals from the input fibers 10 are incident on aligned mirrors 1. The mirrors 1, with the aid of an auxiliary mirror 12 and appropriate imaging lenses 13, are electrically controlled to reflect the incident optical signals to respective output fibers 11. In alternative schemes, the input fibers and the output fibers are in separate arrays, and a pair of MEMs mirror arrays are used to perform the cross connect function.
The tilting of each mirror is controlled by applying specific electric fields to one or more of the electrodes (not shown) beneath the mirror. Undesirable variations in the gap spacing between the mirror layer and the electrode layer alter the electric field, which affects the degree of mirror tilting. This in turn alters the path or coherency of light signals reaching the receiving fibers, thus increasing the signal loss during beam steering.
A MEMs device is essentially composed of two layers: a component layer comprising the array of microscale components, such as microscale mirror elements, movably coupled to a surrounding frame and an actuator layer comprising the microscale actuators, such as electrodes, and the conductive paths needed to move the components. Microscale components typically have a maximum dimension of less than 10,000 micrometers. One approach to fabricating the array is to fabricate the actuator layer and the component layer as successive layers on the same workpiece and then to lift up the component layer above the actuator layer using vertical thermal actuators or stresses in thin films. This lift-up process is described in U.S. patent application Ser. No. 09/415,178 filed by V. A. Aksyuk et al. on Nov. 8, 1999 and assigned to applicant""s assignee.
An alternative approach is to fabricate the component layer on one substrate, the actuator layer on a separate substrate and then to assemble the two substrates as mating parts with accurate alignment and spacing. The two-part assembly process is described in U.S. Pat. No. 5,629,790 issued to Neukermans et al. on May 13, 1997 and in U.S. patent application Ser. No. 09/559,216 filed by Greywall on Apr. 26, 2000, both of which are incorporated herein by reference. This two-part assembly provides a more robust structure, greater component packing density and permits larger component sizes and rotation angles as well as scalability to larger arrays.
To retain the accurate lateral alignment of the component layer and the actuator layer once the alignment is achieved, often requires high temperature bonding processes such as soldering at xcx9c100-300xc2x0 C., epoxy curing at 100-200xc2x0 C., polyimide curing at xcx9c250-400xc2x0 C., glass frit bonding (sometimes called glass solder bonding) at 400-700xc2x0 C., or anodic bonding at 400-900xc2x0 C. But the exposure of the MEMs components to temperatures even as low as xcx9c150xc2x0 C. can cause undersirable distortion or curvature. If the components are mirrors, heat can also cause metallurgical reactions at the interfaces between the mirror metallization and the silicon substrate with consequent contamination of the mirror metal, creep and dimensional changes, formation of brittle intermetallic compounds, and long-term reliability problems. The bowing or curving of the mirrors generally results in non-focused or non-parallel light reflection and loss of optical signal. Accordingly, there is a need for an assembly process that can be carried out at ambient temperature without having to expose the MEMs device to high temperature.
In accordance with the invention, a MEMs device comprises a component layer, an actuator layer and an intervening spacer. The component layer, the spacer and the actuator layer are assembled at ambient temperature and held together in lateral alignment by resilient spring members. The spacer provides the walls of a cavity between a component and an actuator to permit movement of the component. The walls are advantageously conductive and cover the bulk of the peripheral boundary of the cavity to provide electrostatic isolation and aerodynamic isolation.